1. Technical Field
The present invention relates to a developing device and a developing method, which develop an electrostatic latent image formed on an image supporting body, by a liquid developer including toner and carrier liquid, and an image forming apparatus which forms an image by transferring a developer image developed by a developing device to a recording medium and fixing the image.
2. Related Art
There have been proposed a variety of image forming apparatuses which develop and visualize an electrostatic latent image by using a liquid developer of high viscosity, in which toner composed of solid components is dispersed in liquid solvent serving as carrier liquid. The developer used in the image forming apparatus is a developer in which solid components (toner particles) are suspended in organic solvent (carrier liquid) of high viscosity which is composed of silicon oil, mineral oil, edible oil, or the like and has electrical insulation properties. As for the toner particle, an extremely minute particle having a particle diameter approximately of 1 μm is used, thereby being able to achieve higher image quality in comparison with conventional dry type image forming apparatuses which use a toner particle having a particle diameter of 7 μm.
As for an image forming apparatus which uses such a liquid developer, for example, in JP-A-2002-99151, there is disclosed an image forming apparatus which sought for the improvement in development efficiency by uniformly coating a liquid developer on a developing roller through the adjustment of the contact pressure of a blade coming into contact with an anilox roller or the adjustment of the rotary speed of the anilox roller.
Also, in JP-A-2002-287513, there is disclosed an image forming apparatus in which an intermediate roller is provided between a coating roller and a developing roller and the intermediate roller and the coating roller are rotated at the same speed and in the same direction as each other. According to this image forming apparatus, since the developing roller does not come into direct contact with the coating roller, abrasion and damage due to a recessed portion formed in the surface of the coating roller do not occur, so that the developing roller suffers reduced mechanical stress, whereby the life of the developing roller can be prolonged.
In this manner, in the image forming apparatuses which use a liquid developer, a coating roller (anilox roller) having a recessed portion in its surface is used in order efficiently to scoop up the liquid developer and also precisely to measure the liquid developer.
However, in a case where the coating roller (anilox roller) with a recessed portion formed in the surface in this way is used, the transfer pattern of the liquid developer by the recessed portion appears, thereby leading to unevenness in the film thickness of the liquid developer. In particular, in JP-A-2002-287513, as can be seen in the graph of FIG. 8 thereof showing the relationship between the roller velocity ratio of the developing roller and the intermediate roller and the coating amount of the liquid developer which is coated on the developing roller, a region where the roller velocity ratio is 1.0 or less becomes a non-uniform region, and therefore cannot be used for image formation.
In an image forming apparatus, the resolution of an image formed can be improved by reducing the coating amount of the developer which is coated on the developing roller (by thinning the film thickness of the liquid developer). However, in the image forming apparatus disclosed in JP-A-2002-287513, since the lower limit of the coating amount, that is, the film thickness of the liquid developer is restricted, higher resolution cannot be realized.
In addition, in a developing device, it is necessary to change the amount of a supplied liquid developer according to the kind of a paper or required image density on a paper. For example, in a coarse paper such as a rough paper, in order to obtain the same on-paper concentration as that on a plain paper, it is necessary to supply more liquid developer. On the other hand, in a coated paper (smooth paper), the same on-paper concentration can be realized by the supply of the amount of a liquid developer which is less than that in a plain paper. If a broad control range of the film thickness of a liquid developer can be secured so as to be able to enlarge the range of the supply amount of the liquid developer, printing on various kinds of papers can be assured and it is also possible flexibly to accommodate to the change of concentration.